Recent years have seen tremendous growth in the number and variety of display devices available to the public. Computers (whether desktop, laptop, or notebook), personal digital assistants (PDAs), mobile phones, and thin LCD TVs are but a few examples. Although some of these devices can use ordinary ambient light to view the display, most include a light panel referred to as a backlight to make the display visible.
Many such backlights fall into the categories of “edge-lit” or “direct-lit.” These categories differ in the placement of the light sources relative to the output area of the backlight, where the output area defines the viewable area of the display device. In edge-lit backlights, one or more light sources are disposed along an outer border or edge of the backlight construction outside the zone corresponding to the output area. The light sources typically emit light into a light guide, which has length and width dimensions on the order of the output area and from which light is extracted to illuminate the output area. In direct-lit backlights, an array of light sources is disposed directly behind the output area, and a diffuser is placed in front of the light sources to provide a more uniform light output. Some direct-lit backlights also incorporate an edge-mounted light, and are thus illuminated with a combination of direct-lit and edge-lit illumination.
One important aspect of an edge-lit backlight is that the light illuminating the display should be uniformly bright. Illuminance uniformity is particularly a problem when the light sources used at the edge of the backlight are point sources, e.g., light emitting diodes (LEDs). In such cases, the backlight is required to spread the light across the display panel so that the displayed image lacks dark areas. In addition, in some applications, the display is illuminated with light from a number of different LEDs that produce light of different colors. Because the human eye more easily discerns variations in color than in brightness, it can be difficult to effectively mix light sources that produce different colors to provide white illumination light to the display. It is important in these situations that the light from the different LEDs be mixed so that the color, as well as the brightness, are uniform across the displayed image.